TheOtherEric

Is there a procedure spelled out anywhere for testing the operation of the LSD? I have a 74k mile car in a race shop for a PPI and since I'll be doing a lot of track events, I figure this would be a good time to check the LSD. Plus, if it's shot, the car is under extended warranty so the seller said he'd make sure anything gets fixed!

I checked the factory worshop manuals but didn't see anything, nor did my searches here yield anything. Any help?

Thanks!

vjd3

The esteemed Dr. Sandoz tested it by getting on his bicycle and going for a ride after ingesting a massive dose of the compound he had just invented ... oops, wrong LSD ;-)

I think you need access to an axle dynamometer to test it, there is probably a procedure in the workshop manuals; there is one for testing the C4 AWD and viscous clutch.

Vic
95 C4 cab

TheOtherEric

Possibly, but I sure couldn't find it. I did find this thread in which Steve Weiner states that you can "unbolt the CV joints from the flanges on each side of the tranny and measure the breakaway torque, flange-to-flange". However, I don't know what breakaway torque values to expect.

https://rennlist.com/forums/993-forum/137084-limited-slip-diff-there-or-not.html

chris walrod

I believe (I'll check tonight), the service manual states something to that effect, diff break-away torque. However, not sure if the service manual states a service limit?

wombat7

Heres an easy way to check and see if your LSD is working, lift the rear end of the car into the air. Turn one of the rear wheels, if the opposite wheel turns in the opposite direction, your LSD is working, if it turns in the same direction, the LSD is shot

Ron

Are you sure you don't have that backwards. Both wheels should turn in the same direction, I think, for LSD to be working properly.

TheOtherEric

My understanding, based solely upon comments in some prior threads, is that this test is not actually very telling for a 993. Right out of the box, apparently the "spin one wheel test" might be useful, HOWEVER, prior threads indicate that if the opposite wheel turns in the same direction, the LSD is GOOD. At any rate, this doesn't seem to be a useful test for most of us.

TheOtherEric

Here are some older threads about testing LSD's for the curious:
https://rennlist.com/forums/993-forum/93758-update-on-c4s-awd-and-lsd-function-test.html
https://rennlist.com/forums/993-forum/15037-newby-993-question-about-lsd.html

I'm surprised nobody has suggested either the "one wheel in the dirt" or the "clutch drop/ skid mark" methods yet!

Crimson Nape Racing

Ron is correct. If you have a LSD, the tires will rotate in the same direction. As to whether or not that test will determine if the LSD is good or not... I be not knowin.

wombat7

Yeah sorry about the mistake guys.. I had a brain fart or something. about the direction of the wheel spin

ScottMellor

I usually just wait for a little rain, and then e-a-s-e the clutch out to preserve my delicate 1rst gear, and then I nail it. If the car just stays stationary and I hear spinning and whirring noises at both ends, I figure my LSD is still okey dokey.

Ray Calvo

I always thought that if you jack up one rear tire, have the car in 1st gear, and try to turn the wheel that's up in the air, if you can't turn it you've got functional LSD. If you can turn it, you don't have LSD or it's acting like it's Woodstock namesake.

wombat7

Sorry for the hijack

Ray... where in Pittsburgh do you live?? I'm from about an hour and a half north in meadville

chris walrod

If one has the car in gear, and one tire on the ground, by spinning the wheel in the air, you will be turning the engine over. With both wheels in the air and in gear, the wheel will spin the opposite direction. With an LSD equipped car, in neutral, the opposite tire -should- spin the same direction.

If the wheels spin the opposite direction with LSD and the car in neutral, doesnt necessarily mean the LSD is shot, its just that the clutch plates have worn somewhat and the diff has lost some of its preload. Not ideal, but the LSD will function when torque is applied, although it will take more torque to lock up. This is the quantitative value we are after, so to speak.

Aside from Torsen diffs, our LSD equipped 993's have the ramp type clutch actuation differential. There is two ramps that have asymetrical angles that determine how much lock-up for accel and decel.

I feel, the LSD benefits the 993 under braking more so than with accelleration.

1999Porsche911

Hey Eric. You are in Chicago. There is plenty of snow and ice around here so take the car to a side road and put one of the back wheels on the ice covered shoulder. Put the car in gear and start to move. If the LSD is working, the road side tire will get traction and off you go.